'03-older Eden WT550 vs. '04 and newer

This is pretty much the same amp, the only thing I see that changed is the power rating at 2 ohms. It used to be rated at 620 watts RMS into 2 ohms, and starting in late '03 was rated at 750 watts. Was this just a change in rating, or did something change with the amp? 8 ohm and 4 ohm power is the same. So I'm guessing they may have come up with a different way of rating these so they gave out better figures? I searched and didn't find anything on this difference. Anyone know the deal here? Thanks